1. Field of the Invention
The present invention relates to an electrostatic discharge protection circuit device used for preventing an over-current due to, for example, a surge voltage from flowing into an internal circuit of a semiconductor device.
2. Description of the Related Art
A semiconductor device of a semiconductor integrated circuit is provided with an electrostatic discharge (ESD) protection circuit to protect the internal circuit from an over-current due to, for example, a surge voltage.
ESD protection circuits having various structures have been proposed, and the following structure is widely used. According to the structure, the internal circuit is protected using a diode or a MOS transistor. However, with high integration and low voltage of the semiconductor device, the internal circuit is not sufficiently protected even if the foregoing diode or MOS transistor are used. For example, PCT National Publication No. 2004-531055 discloses the following ESD protection circuit. The ESD protection circuit uses a silicon controlled rectifier (SCR) as a protection element. The SCR makes a switching operation at high speed, and a large current is carried through there. In addition, the SCR has strong durability with respect to breakdown. Thus, the ESD protection circuit using the SCR has excellent properties such as high performance and high protection ability. When a surge voltage is applied, the voltage at which the SCR turns on to start protecting the internal circuit from over-current is generally called the trigger voltage.
In the ESD protection circuit disclosed in the foregoing Publication, the trigger voltage is determined in the design stage. Thus, it is difficult to control the voltage after the ESD protection circuit is manufactured. Moreover, if the ESD protection circuit is used for a certain purpose, there is a need of making almost no protection operation of the ESD protection circuit. However, the ESD protection circuit disclosed in the foregoing Publication cannot effect the operation described above.